The present invention relates to an electrophotographic image forming apparatus, an electrostatic recording type image forming apparatus, and a developing agent used in these apparatuses and, more particularly, to a two-component developing agent containing magnetic toner and carrier and a two-component developing type image forming apparatus.
Generally, when magnetic toner is manufactured by mechanical grinding, the viscosity during melt kneading becomes higher than that of nonmagnetic toner owing to the influence of a magnetic powder added. If the viscosity is too high, an excess shearing force acts on a binder resin to break the resin chain, resulting in easy changes in the resin characteristics. In the manufacture of conventional toner, therefore, in the melt-kneading step kneading must be performed by weakening the shearing force, e.g., lowering the speed of a kneading screw. That is, kneading must be performed by lowering the productivity.
The bulk density of the magnetic powder used is preferably as low as possible because variations in the magnetic characteristics decrease. However, the smaller the bulk density of a powder such as a magnetic powder, the more easily the powder flocculates. So, the magnetic powder cannot easily get intimate with a viscous material such as a binder resin. This makes kneading difficult. When a magnetic powder having low bulk density is subjected to melt-kneading together with a binder resin, the screw cannot well bite into toner particle materials, and this lowers the kneading process amount. It is difficult to improve this lowering of the process amount by the manufacturing conditions. Furthermore, the addition of a small amount of a material (e.g., CCA) other than the magnetic powder also worsens biting of the screw, causing a lowering of the process amount.
For these reasons, the productivity is conventionally ensured by the addition of a magnetic powder having high bulk density. However, the higher the bulk density of a magnetic powder, the larger the variations in the magnetic characteristics between the individual magnetic powder particles. Additionally, uniform dispersion of the magnetic powder is difficult to obtain in a binder resin, so stable magnetic characteristics cannot be obtained. This deteriorates image characteristics such as background development.
Accordingly, it is difficult to achieve both high productivity and good image characteristics only by the bulk density of a magnetic powder.
The present invention has been made in consideration of the problems of the above prior art, and has as its first object to provide a developing agent having superior magnetic characteristics and high productivity and capable of forming high-quality images.
It is the second object of the present invention to provide an image forming apparatus which uses a developing agent having superior magnetic characteristics and high productivity and which can form high-quality images.
According to the first aspect of the present invention, there is provided a developing agent containing toner particles obtained by melting and kneading a particulate binder resin and a magnetic powder to form a mixture, and then milling the mixture, wherein said particulate binder resin material has an average particle size of 250 to 600 xcexcm and the magnetic powder has an apparent bulk density of not less than 0.3 g/cm3 and a compacted bulk density of not more than 2.5 g/cm3.
According to the second aspect of the present invention, there is provided a method of manufacturing a developing agent, comprising the melt-kneading step of obtaining a kneaded product by melting and kneading a particulate binder resin material having an average particle size of 250 to 600 xcexcm and a magnetic powder having an apparent bulk density of 0.3 g/cm3 or more and a compacted bulk density of 2.5 g/cm3 or less, and the milling-classification step of obtaining toner particles by milling and classifying the kneaded product.
According to the third aspect of the present invention, there is provided an image forming apparatus comprising a developing unit, transfer unit, and cleaning device arranged in this order around, and opposite to, an image carrier, the developing unit containing a developing agent containing toner particles obtained by melting and kneading a particulate binder resin and a magnetic powder to form a mixture, and then milling the mixture, and carrier, and comprising a developing roller which comprises a hollow cylindrical rotary sleeve for carrying the developing agent and a magnet roll having a plurality of magnetic poles and capable of rotating independently of the sleeve, and a fixing unit placed downstream of the transfer unit and having a pair of fixing rollers.
wherein said particulate binder resin material has an average particle size of 250 to 600 xcexcm and the magnetic powder has an apparent bulk density of not less than 0.3 g/cm3 and a compacted bulk density of not more than 2.5 g/cm3.
According to the present invention, the manufacture of the developing agent and the dispersion of the magnetic powder are facilitated, and an excellent image can be formed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.